1. Field of the Invention
The present invention relates to a semiconductor device and manufacturing method thereof.
2. Description of the Background Art
Recently, in view of saving energy, an inverter circuit has come to be widely used for controlling home electric appliances and industrial power apparatuses. In an inverter circuit, power control is effected by repeating on and off of voltage or current, using a power semiconductor device. When rated voltage is 300 V or higher, an insulated gate bipolar transistor (IGBT) is mainly used, considering its characteristics.
It is often the case that the inverter circuit drives an inductive load such as an induction motor. In that case, back electromotive force generates from the inductive load. Therefore, a free wheel diode for circulating a current generated by the back electromotive force opposite in direction to the main current of IGBT, becomes necessary.
A common inverter circuit having an IGBT and the free wheel diode as separate parts connected in parallel has been used. In order to reduce size and weight of the inverter device, however, a semiconductor device having the IGBT and the free wheel diode integrated as one chip has been developed.
Conventionally, a semiconductor device having the IGBT and the free wheel diode integrated as one chip as described above has been proposed, for example, in Japanese Patent Laying-Open Nos. 05-152574, 06-085269 and 07-153942.
When the free wheel diode of the semiconductor device described above is in a forward conductive state, a large number of carriers for carrying current are stored in the semiconductor device. Therefore, immediately after switching of the current to the reverse direction, a phenomenon in which a large current flows instantaneously in the reverse direction (recovery operation) occurs.
According to the conventional art described above, in the recovery operation, a current may possibly flow to cause carriers stored in a region around an outer periphery of IGBT and free wheel diode forming regions (cell region) to concentrate at an end portion of the cell region, causing breakdown of the semiconductor device (recovery breakdown).
When recovery breakdown occurs, the semiconductor device can no longer function well as an IGBT or a free wheel diode, because, for example, a simple resistor in electric equivalence is formed between electrodes. Further, failure such as melting of metal electrode may sometimes be observed. Crystal structure analysis of the end portion of cell region sometimes reveals destruction of original single-crystal structure.
It is noted that the longer the carrier life time in a semiconductor device, the larger the current that flows in the recovery operation, and the higher the possibility of recovery breakdown. Though the carrier life time can be made shorter by electron beam irradiation, for example, such an additional process step increases manufacturing cost.